[mod] replace /help by /info pages and include pages in project docs

This patch implements a bolierplate to share content from info-pages of the
SearXNG instance (URL /info) with the project documentation (path /docs/user).

The info pages are using Markdown (CommonMark), to include them in the project
documentation (reST) the myst-parser [1] is used in the Sphinx-doc build chain.

If base_url is known (defined in settings.yml) links to the instance are also
inserted into the project documentation::

    searxng_extra/docs_prebuild

[1] https://www.sphinx-doc.org/en/master/usage/markdown.html

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

searx/templates/simple/base.html

@@ -58,7 +58,7 @@
   </main>
   <footer>
     <p>
-    {{ _('Powered by') }} <a href="{{ url_for('help_page', pagename='about') }}">searxng</a> - {{ searx_version }} — {{ _('a privacy-respecting, hackable metasearch engine') }}<br/>
+    {{ _('Powered by') }} <a href="{{ url_for('info', pagename='about', locale=current_locale) }}">searxng</a> - {{ searx_version }} — {{ _('a privacy-respecting, hackable metasearch engine') }}<br/>
         <a href="{{ searx_git_url }}">{{ _('Source code') }}</a> |
         <a href="{{ get_setting('brand.issue_url') }}">{{ _('Issue tracker') }}</a> |
         <a href="{{ url_for('stats') }}">{{ _('Engine stats') }}</a> |